avoid writing attributes unnecessarily on content tags

refs CNVS-15830

test plan:
 * add an item to a module
 * it should work

Change-Id: I5618b3f7293223aed95f54f05c40a3c0ca646b65
Reviewed-on: https://gerrit.instructure.com/42860
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>